1. Field of the Invention
The present invention relates to a frequency generator of the type in which a pulse signal corresponding to the rotational speed of a motor is generated and also relates to a miniature motor provided with such frequency generator.
2. Description of the Prior Art
Frequency generators used for detecting the rotational speed of a motor and further for controlling the detected speed have been known in the art.
The principle structure of frequency generators is that a closed magnetic path including a permanent magnet is provided in such manner that the magnetic reluctance of the closed magnetic path may vary in proportion as the rotational speed of the rotary shaft and within the space enclosed by the closed magnetic path there is disposed a generation coil producing a pulse signal in response to a change of magnetic flux caused by the variation of the magnetic reluctance.
A typical example of such frequency generator pertinent to the present invention is disclosed in Japanese Patent Application Publication No. 6910/1963. The frequency generator disclosed in this publication includes a permanent magnet in a form of center bored disc. The disc like permanent magnet is so magnetized as to have one of the magnetic poles at the circumferential edge portion of the disc and another at the center portion of the disc. At the circumferential magnetic pole side, the permanent magnet disc is connected with a stationary frame made of magnetically conductive material. A magnetically conductive rotor rotatable with the rotary shaft is so disposed that the front end portion of the rotor is close to and opposed to the center magnetic pole side of the permanent magnet.
the frequency generator further comprises a first and a second group of projections. The first group of projections comprises a plural number of magnetically conductive projections and is rotatable with the rotor. The second group of projections comprises the same number of magnetically conductive projections as that of the first one and is fixedly supported by the stationary frame in such manner as to surround the first group of projections. For the above described type of permanent magnet, a closed magnetic path is formed by the rotor, the first and second groups of projections and the stationary frame. Within the inner space of the close magnetic path there is disposed a power generation coil. As the rotary shaft rotates, a change of magnetic reluctance occurs between the first group of projections and the second one in proportion as the rotation of the rotary shaft. Thus, a corresponding change of magnetic flux is caused by the change of the magnetic reluctance and thereby in the power generation coil there is produced a pulse signal in response to the change of magnetic flux.
The prior art frequency generator of the above described arrangement has the following disadvantage.
First of all, the permanent magnet used therein as a source of magnetomotive force is very difficult to manufacture and very expensive. The permanent magnet is in a shape of center bored disc and is magnetized radially along the direction of radius of the disc. Namely, the direction of magnetization is so selected that the magnet disc has one of the magnetic poles at the circumferential edge portion of the disc and another pole at the center portion thereof. Such a permanent magnet is very difficult to manufacture and also very expensive.
Secondly, the quantity of electric power generated in the generating coil is too small to obtain a good detection signal. Since the magnetomotive force of the permanent magnet is limited by the quantity of magnetic charge of the magnetic pole at the center portion of the disc, the magnetomotive force is very weak and therefore the quantity of magnetic flux flowing the magnetic path necessarily becomes small. As a result, the electric power generated in the generating coil at the time of of motor speed becomes too small to obtain a good detection signal. In this case, of course, it is possible to prevent such a reduction of the magnetomotive force to some extent by enlarging the thickness of the permanent magnet disc. However, this means the use of a larger size of permanent magnet, which is against the general requirement of miniaturization and compactness of frequency generator as a whole. If the size of frequency generator is unduely enlarged, then it will give arise a difficulty in mounting the generator on a common type of miniature motor.
Lastly, the prior art frequency generator has a limitation in its application. When the rotary shaft has any play of movement in the thrust direction, the distance of air gap between the front end of the rotor and the center magnetic pole side of the permanent magnet varies from time to time. Therefore, the amount of magnetic flux flowing through this area is also varied. As a result, there is caused a misleading change in the quantity of electric power generated in the generating coil. To avoid this trouble, means for preventing the thrust play of the rotary shaft must be provided additionally. Otherwise, the application of the frequency generator must be limited only to such type of motor the rotary shaft of which has no play of movement in the thrust direction.